1. Technical Field
The present disclosure relates to a device for detecting and measuring the distance from an object, based on a propagation time of a photon beam emitted in the form of pulses and reflected on the object. The present disclosure applies in particular to touch screens such as those used in mobile phones, and to the detection of an object near such a screen.
2. Description of the Related Art
It is known to use photodiodes as a distance detection and measurement element, using an avalanche phenomenon which may occur in the PN junctions of photodiodes. An avalanche phenomenon may occur in a diode PN junction when the diode is reverse biased near the junction breakdown voltage. This phenomenon can be used in two ways in an avalanche photodiode. If the avalanche photodiode is reverse biased just below the breakdown voltage, the photodiode then generates an electric current proportional to the intensity of the photon flux received by the photodiode, with a gain of some hundreds with a semiconductor such as silicon.
To detect low intensities of photon flux, it is known to use photodiodes which may be reverse biased above the breakdown voltage. Such photodiodes are called Single Photon Avalanche Diode (SPAD) or diodes operating in “Geiger” mode. Each time such a photodiode receives a photon, an avalanche phenomenon occurs in the photodiode PN junction, generating an intense current. To avoid destructing the photodiode by this intense current, the photodiode is connected to a quenching circuit allowing the avalanche process to be stopped some nanoseconds after appearing.
To perform a distance measurement, it is known to light a detection area with a pulsed light source such as a pulsed laser source, and to detect photons reflected by an object present in the detection area using a set of SPAD photodiodes. The distance from the object in the detection area is evaluated based on the propagation time or Time Of Flight (TOF) between the transmit time of a light pulse and the receipt time of a pulse across a photodiode, resulting from the photodiode avalanche triggering. The measure accuracy depends in particular on the duration of the light pulses emitted by the source; the shorter these pulses, the more accurate the measurement.
In applications to the detection of an object near a touch screen, the photodiodes are placed under a plate transparent to the photons to be detected. The plate may reflect photons emitted by the source directly towards the photodiodes, which disturbs the distance measurement. Indeed, if an object is at a distance D from the photodiodes and if the plate is at a distance d from the photodiodes, the photodiodes are going to detect photons from a reflection of the photon beam emitted on the object and also on the plate. If the measure of a distance is evaluated on the basis of an average of the photodiode triggering times, the obtained measure will be around (D+d)/2, i.e., half the distance D if the distance d is small. If there is no object in the detection area, the detection device supplies an incorrect distance measure equal to d.
To reduce the influence of reflections on the plate of photons emitted by the source, the photon beam transmit unit may be moved away from the photodiodes. However, this arrangement can reduce a detection area which coincides with the largest possible area lighted by the source.